In general, (meth)acrylic acids are separated or purified by a distillation method.
In recent years, for the purposes of improving a separation efficiency by distillation, increasing a throughput from distillation, etc., high-performance fillers have been developed and now used in distillation columns for various processes. However, since the (meth)acrylic acids are extremely readily polymerized, the conventional tray-type distillation columns, especially high-performance packed columns, still suffer from significant problems such as undesired production of polymers within these distillation columns.
Hitherto, as the method for preventing the production of polymers of the (meth)acrylic acids, there have been proposed the method of improving a tray structure of a distillation column (Japanese Patent Application Laid-Open (KOKAI) No. 2000-300903), the method using a specific polymerization inhibitor (Japanese Patent Application Laid-Open (KOKAI) No. 7-53449) and the like. However, these conventional methods still fail to allow a continuous operation of the distillation column for a long period of time, and require periodic inspection, washing and repair thereof, etc., which must be accompanied with stopping of its operation. The polymers are frequently produced from the beginning of operation of the distillation column. Once such polymers are produced, a stable flow of gases or liquids within the distillation column is disturbed, thereby frequently causing such a phenomenon that production of the polymers is further promoted.
Some of the above distillation columns are equipped with a reboiler (heat exchanger for heating) for heating a part of a bottom liquid removed from the distillation column and then returning the thus heated liquid back to a bottom of the distillation column. In addition, some of the distillation columns may be equipped with a reflux tank for condensing a distillate removed from a top of the distillation column and then returning a part of the resultant condensate to the distillation column in order to enhance a recovery percentage and purity of aimed products distilled.
Conventionally, upon stopping an operation of the above-described reduced-pressure type distillation column, the heating of the reboiler is interrupted, and an inside of the distillation column is returned to an ordinary pressure by feeding an inert gas such as nitrogen thereinto.
However, in the conventional methods of stopping the operation of the above reduced-pressure type distillation column, the inert gas such as nitrogen is fed into the distillation column before the inside of the distillation column reaches a sufficiently low temperature, namely under such a condition that the distillation column is still filled with a large amount of vapors of the (meth)acrylic acids containing no polymerization inhibitor. Therefore, the (meth)acrylic acids tends to undergo polymerization to produce polymers thereof. The removal of the polymers thus produced within the distillation column requires complicated procedures, resulting in problems such as increased maintenance costs and prolonged working time upon its periodic maintenance or repair.
Meanwhile, when the inside of the distillation column is returned to an ordinary pressure by feeding a gas having a polymerization-inhibiting effect such as air into the distillation column in order to prevent the occurrence of polymerization reaction upon stopping the operation of the distillation column, an inside atmosphere of the distillation column may fall within an explosion range. Therefore, such a method cannot be adopted for the above purpose. Further, in the case where the inert gas is fed after the inside temperature of the distillation column is sufficiently lowered, it takes a long period of time until sufficiently cooling the distillation column, i.e., until feeding the inert gas thereinto, resulting in poor operation efficiency.
Meanwhile, it will be suggested that after discharging a whole bottom liquid in the distillation column, the liquid stored in the reflux tank is refluxed thereinto to cool an inside of the distillation column and then return an inside pressure thereof to an ordinary pressure. However, only the liquid in the reflux tank is insufficient to cool the inside of the distillation column. In addition, when the high-temperature bottom liquid is transferred into a tank, undesirable polymerization of the (meth)acrylic acids may be induced due to temperature rise in the tank.
On the other hand, as the above periodic washing and repairing method, Japanese Patent Application Laid-Open (KOKAI) No. 2000-319223 has proposed the method of washing a distillation column with a base solution containing a base substance such as sodium hydroxide and potassium hydroxide, and then washing with a solvent (more preferably water). However, according to the present inventors' knowledge, this method fails to remove polymers and solids precipitated in the column to a sufficient extent.
More specifically, the solids derived from the (meth)acrylic acid and/or (meth)acrylic esters is enhanced in absorption property thereof by the alkali substance, and swelled up even with a slight amount of water. The water-swelled solids (swelled solid matters) exhibit a stickiness (under gelled condition) and, therefore, is not easy to separate and peel from portions onto which the solids are adhered or deposited. Also, at the portions of the distillation column to which the aqueous alkali solution is hardly reached, such as a rear side of respective trays, it may be difficult to remove the solids adhered or deposited thereto.
The present invention has been attained to solve the above problems. Objects of the present invention are as follows:
(1) To provide a method for preventing polymerization of monomers frequently caused upon separation and purification of (meth)acrylic acids. In particular, to provide a method of keeping an inside of a distillation column in such an atmosphere in which the (meth)acrylic acids are hardly polymerized, upon starting or re-staring the distillation procedure.
(2) To provide a method for early and safely stopping an operation of the distillation column in which the polymerizable (meth)acrylic acids are distilled under reduced pressure, by preventing the polymerization within the distillation column.
(3) To provide a method for washing the distillation column in which crude (meth)acrylic acids are separated and purified. In particular, to provide a method for efficiently washing the distillation column while preventing impurities from being mixed in the (meth)acrylic acids and recovering useful materials by using various substances obtained from processes conducted before or after the distillation column, in the process for production of the (meth)acrylic acids.
(4) To provide a method for preventing polymerization of monomers frequently caused upon separation and purification of an acrylic monomer. In particular, to provide a method of keeping an inside of a distillation column in such an atmosphere in which the acrylic monomer is hardly polymerized, upon starting or re-staring the distillation procedure.
(5) To provide a method for handling the above compounds which is improved so as to efficiently wash out and remove solids derived from (meth)acrylic acid and/or (meth)acrylic esters, adhered onto the surface of a handling device including conduits.